Environmental regulations are becoming increasingly strict with regard to engine exhaust emissions such as nitrogen oxides (NOx) and particulate matter. More stringent environmental regulations with regard to diesel engine particulate emissions has warranted the need for diesel particulate filters and/or other exhaust aftertreatment devices, such as NOx adsorbers, to be placed in the exhaust gas stream for removing or reducing harmful exhaust emissions before the exhaust is permitted to enter the atmosphere.
Typically, exhaust aftertreatment systems must initiate and control regeneration of the particulate filters, NOx adsorbers, and other exhaust treatment devices from time to time as the devices fill up with soot, NOx, or the like. Regenerating the devices removes some or all of the particulates built up on the devices by oxidizing the particles. As an example, regeneration of a particulate filter is done by increasing the temperature of the filter to a level where the soot is oxidized, e.g., above 400° C., and maintaining that temperature for a desired period of time, e.g., several minutes or longer, depending on circumstances including the size of the filter, the amount of soot on the filter, the uniformity level of the soot, etc.
The temperature of the filter is increased by increasing the temperature of the exhaust gas passing through the filter by any of various techniques known in the art. Although increasing the exhaust gas temperatures can effectuate the positive and desirable result of regenerating an exhaust aftertreatment device, exhaust gas temperatures during such regenerating events can reach extreme levels, e.g., 650° C. or more, possibly causing undesirable side effects. For example, the high exhaust temperatures required for filter regenerations usually means the exhaust leaving the tailpipe of the vehicle is much hotter than it would be during normal operation, particularly at stationary or low-speed operation. This creates a potential safety hazard with regard to the heat flux of the gases leaving the tailpipe and creating discomfort or injury to humans, animals, or plants in proximity. Moreover, extreme exhaust gas temperatures resulting from regeneration events can increase the surface temperature of exhaust train components, increase the risk of fire hazards, and cause damage to street surfaces and other objects. Additionally, extreme exhaust gas temperatures can discolor, e.g., blacken, tailpipe components, especially tailpipes with chromed outer surfaces.
Several approaches have been employed for mitigating heat from an exhaust gas stream to reduce the temperature of the exhaust as it exits the tailpipe. For example, some fire trucks are equipped with a water spray device at the exhaust outlet for exhaust cooling, but such a scheme is limited to a situation where there is a ready water supply as well as experienced firefighters. Other approaches include exhaust diffusion devices coupled to the outlet of the tailpipe. The diffusion devices are configured to cool the exhaust gas leaving the tailpipe by diluting and dispersing the exhaust gas. However, such diffusion devices are not designed to cool the exhaust gas at the tailpipe outlet, but rather to reduce the temperature of the exhaust gas at a regulated distance, e.g., six inches, away from the outlet of the tailpipe to a temperature below a regulated maximum temperature. Although some conventional diffusion devices are successful at achieving a desirable mitigation of exhaust gas heat outside of the tailpipe, e.g., at a distance away from the tailpipe outlet, such devices do not achieve cooler exhaust gas temperatures at the tailpipe outlet. Accordingly, the temperature of exhaust at the tailpipe outlet is still extremely high, which can be dangerous to people and objects near the tailpipe and cause bluing or blackening of the tailpipe itself.
To achieve cooler exhaust gas temperatures at the tailpipe outlet before, during, or after regeneration events, exhaust aspirators positioned upstream of the tailpipe outlet have been developed to entrain ambient air into the exhaust gas stream before the exhaust gas exits the tailpipe. Ambient air is entrained into the exhaust gas stream by creating an exhaust pressure drop within the aspirator that causes a vacuum effect to suck in the ambient air. The pressure drop is created by accelerating the exhaust gas through a nozzle and allowing the exhaust gas to expand upon exiting the nozzle. Typically, the pressure drop must be below a certain threshold (e.g., below 1 inch Hg) to prevent harmful levels of engine backpressure. The ambient air is mixed with the exhaust gas stream and, being cooler than the exhaust gas, reduces the temperature of the exhaust gas before it exits the tailpipe. Accordingly, the temperature of the exhaust gas is cooled within the tailpipe.
Conventional exhaust aspirators suffer from several drawbacks however. Generally, the less the ambient air and exhaust gas is mixed within the aspirator or tailpipe, the higher the radial temperature gradient, and the lower the exhaust gas temperature uniformity at the tailpipe outlet. Typically, inadequate mixing results in some portions of exhaust gas being at a generally uniform lower exhaust gas temperature at the tailpipe outlet and some concentrated pockets of exhaust gas remaining at extremely high temperatures. The concentrated pockets can be harmful and cause bluing of the tailpipe. Conventional exhaust aspirators, such as those with a single nozzle, may not adequately mix the entrained ambient air with the exhaust gas to achieve a suitable radial temperature gradient for a given exhaust pressure drop threshold. To achieve better mixing and exhaust uniformity at the aspirator outlet, some conventional exhaust aspirators are lengthened. However, longer aspirators can be more expensive to manufacture due to additional material and can occupy more valuable space within the exhaust system that could be used for other components.
Accordingly, an exhaust aspirator is desired that more adequately mixes entrained air with exhaust gas within a tailpipe to achieve a lower exhaust gas radial temperature gradient and greater exhaust gas uniformity at the tailpipe outlet, less expensive to manufacture, and occupies less space than conventional aspirators.